Messages in this thread | | | Date | Sun, 25 Jun 2023 11:15:01 +0800 | Subject | Re: [PATCH 24/29] mm: vmscan: make global slab shrink lockless | From | Qi Zheng <> |
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On 2023/6/24 19:08, Qi Zheng wrote: > Hi Dave, > > On 2023/6/24 06:19, Dave Chinner wrote: >> On Fri, Jun 23, 2023 at 09:10:57PM +0800, Qi Zheng wrote: >>> On 2023/6/23 14:29, Dave Chinner wrote: >>>> On Thu, Jun 22, 2023 at 05:12:02PM +0200, Vlastimil Babka wrote: >>>>> On 6/22/23 10:53, Qi Zheng wrote: >>>> Yes, I suggested the IDR route because radix tree lookups under RCU >>>> with reference counted objects are a known safe pattern that we can >>>> easily confirm is correct or not. Hence I suggested the unification >>>> + IDR route because it makes the life of reviewers so, so much >>>> easier... >>> >>> In fact, I originally planned to try the unification + IDR method you >>> suggested at the beginning. But in the case of CONFIG_MEMCG disabled, >>> the struct mem_cgroup is not even defined, and root_mem_cgroup and >>> shrinker_info will not be allocated. This required more code >>> changes, so >>> I ended up keeping the shrinker_list and implementing the above pattern. >> >> Yes. Go back and read what I originally said needed to be done >> first. In the case of CONFIG_MEMCG=n, a dummy root memcg still needs >> to exist that holds all of the global shrinkers. Then shrink_slab() >> is only ever passed a memcg that should be iterated. >> >> Yes, it needs changes external to the shrinker code itself to be >> made to work. And even if memcg's are not enabled, we can still use >> the memcg structures to ensure a common abstraction is used for the >> shrinker tracking infrastructure.... > > Yeah, what I imagined before was to define a more concise struct > mem_cgroup in the case of CONFIG_MEMCG=n, then allocate a dummy root > memcg on system boot: > > #ifdef !CONFIG_MEMCG > > struct shrinker_info { > struct rcu_head rcu; > atomic_long_t *nr_deferred; > unsigned long *map; > int map_nr_max; > }; > > struct mem_cgroup_per_node { > struct shrinker_info __rcu *shrinker_info; > }; > > struct mem_cgroup { > struct mem_cgroup_per_node *nodeinfo[]; > }; > > #endif > > But I have a concern: if all global shrinkers are tracking with the > info->map of root memcg, a shrinker->id needs to be assigned to them, > which will cause info->map_nr_max to become larger than before, then > making the traversal of info->map slower.
But most of the system is 'sb-xxx' shrinker instances, they all have the SHRINKER_MEMCG_AWARE flag, so it should have little impact on the speed of traversing info->map. ;)
> >> >>> If the above pattern is not safe, I will go back to the unification + >>> IDR method. >> >> And that is exactly how we got into this mess in the first place.... > > I only found one similar pattern in the kernel: > > fs/smb/server/oplock.c:find_same_lease_key/smb_break_all_levII_oplock/lookup_lease_in_table > > But IIUC, the refcount here needs to be decremented after holding > rcu lock as I did above. > > So regardless of whether we choose unification + IDR in the end, I still > want to confirm whether the pattern I implemented above is safe. :)
Also + RCU mailing list.
> > Thanks, > Qi > >> >> -Dave
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